Q: What kind of advantages does membership of ESO bring?

A: The main advantage for the community of professional astronomers and astrophysicists of an ESO Member State is the availability of observing time on the most advanced ground-based telescopes in the world, as well as access to the southern sky from one of the best observing sites on Earth — the Atacama Desert. Moreover, ESO is the most suitable organisation for conducting mega-science projects in astronomy, and for fostering international astronomical collaborations.

Q:What is ESO's relationship with Chile?

A:ESO located its observatories in Chile because of the exceptional observing conditions to be found in the north of the country which are unequalled anywhere else in the southern hemisphere. There has been a long and fruitful collaboration between ESO and the host country that is celebrating its 50th anniversary in late 2013. More details are available: ESO & Chile (English) / ESO y Chile (Español).

Q: Do ESO observatories only see the southern skies?

A: The region of the sky visible from the Earth changes with latitude. If we could stand at the South Pole, we would only see the southern sky. As we move north towards the equator, the pole goes down from the zenith to the horizon and a larger part of the northern sky becomes visible. ESO observatories are located at a latitude of about 23°S, making the northern sky theoretically visible up to 67°N. Thus 95% of the whole sky is visible from the ESO sites in Chile. As the telescopes cannot point down to the horizon, 87% of the whole sky can be observed by ESO telescopes.

Q: Is light pollution a serious problem for astronomical observations?

A: Yes it is, for optical astronomy. It is well known that only the brightest stars are visible from a city, whilst from very dark places like the ESO sites in Chile it is even possible to see your shadow cast by the brightest stars on a clear moonless night. While visible light pollution does not disturb astronomical observations in other bands of the electromagnetic spectrum such as radio waves, radio astronomy can be affected by terrestrial signals emitted at the same frequencies as those detected by the telescopes.

Q: How much do the Member States pay to run ESO annually?

Q: Is it possible to visit ESO?

A: ESO's La Silla and Paranal Observatories in Chile offer regular scheduled public visits. Media visits to ESO Headquarters in Germany and to the observatories in Chile need to be arranged in advance. For more information, please see: Visiting ESO http://www.eso.org/public/about-eso/visitors/index.html.

Q: Are the colours in the astronomical images real?

A: Unlike commercial digital cameras, the cameras on research telescopes take greyscale images through filters. These are the images used for scientific analysis. For presentation to the general public, observations through multiple individual filters are often combined to produce colour images. For observations made with visible light, we often try to match the colours that our eyes can see. However, often the light that is observed is invisible to our limited human vision, for example, infrared light, and has to be represented with a visible colour instead. Often the shortest wavelength is represented with blue and the longest with red and this choice of colours is called "chromatic". Observations through narrowband filters are usually assigned colours in "chromatic order", although sometimes the ordering is, for aesthetic purposes, not kept chromatic and this leads images known as "enhanced colour images".

Q: Can you give a couple of examples of the latest discoveries made at ESO?

A: That’s quite a challenge; there are plenty to choose from. There are the numerous discoveries on exoplanets made by HARPS on the 3.6-metre telescope at La Silla. HARPS is, without any doubt, the most successful exoplanet hunter in the world. It has recently found several exoplanets that are orbiting in the opposite direction to the rotation of their host star — the exact opposite of what is seen in our own Solar System. The new discoveries provide an unexpected and serious challenge to current theories of planet formation. HARPS has also found the smallest exoplanet to date and made several other firsts.

Also concerning exoplanets, the Very Large Telescope has obtained the first direct spectrum — the “chemical fingerprint” — of a planet orbiting a distant star, thus bringing new insights into the planet’s formation and composition. The result represents a milestone in the search for life elsewhere in the Universe. And the VLT was the first to ever take a direct image of an exoplanet.

On another scale, several of ESO's telescopes were used in a 16-year-long study to obtain the most detailed view ever of the surroundings of the monster lurking at the heart of our galaxy — a supermassive black hole. This black hole can now be studied in great detail, and we have really entered the era of observational black hole physics.

Still further out, the VLT has obtained the spectral signature of the earliest, most distant known object in the Universe, seen only about 600 million years after the Big Bang.

Q: How long is the average life-time of a telescope?

A: It depends very much on the project, its design, objectives and funding. For the whole of ESO’s existence, the most ambitious astronomical projects have demanded ever larger investments that cannot be afforded only by a single country. In order to “recover” such important investments, a facility should operate for at least a couple of decades at the forefront of astronomy, before the next generation of telescopes takes over. After that, the telescopes continue operating, typically with new instruments and often dedicated to specific observing programmes. This is the case with the ESO 3.6-metre telescope, commissioned in 1977. It was for many years the largest European telescope and one of the largest telescopes in the southern hemisphere. It is now home to the world’s foremost exoplanet hunter: HARPS (High Accuracy Radial velocity Planet Searcher).

Q: How many proposals for observing time does ESO get?

A: Each year, about 2000 proposals are made for the use of ESO telescopes, requesting between four and six times more nights than are available. ESO is the most productive observatory in the world, which annually results in an impressive number of peer-reviewed publications: more than two every day of the year.

Q: Why don't we always build the astronomical observatories on the highest mountains, so that the height of the atmosphere is less above the telescopes?

A: The quality of an astronomical site does not necessarily always improve with altitude, as there can be many different meteorological factors involved. These include the stability of the atmosphere, which can, for example, be affected by the terrain and other local conditions. So, while higher altitudes are often good, they are not always better. More practically, at very high altitudes, such as on the 5000-metre Chajnantor plateau where ALMA is being constructed, environmental factors make it difficult to build and operate an observatory. At very high observatory sites, engines lose power in the reduced air pressure, materials must withstand extreme weather conditions, and the low pressure and lack of oxygen affect the human body. Therefore, real scientific benefits are needed to justify the challenges of building an observatory on a higher site.

Q: What is the theoretical maximum resolution of a single telescope?

A: For a single telescope, its resolution (the finest details it can distinguish) is limited by the diffraction of light as it passes through the telescope's aperture. The resolution is improved if the wavelength of the light gets shorter or the telescope diameter gets larger. Since the wavelength may be fixed by the kind of astrophysical observations that are needed, this is one reason why astronomers try to build larger telescopes. This fundamental limit on the sharpness of a telescope's vision is called the diffraction limit, and telescopes that reach it are diffraction-limited.

The sharpness of a telescope's vision may be worse in practice than this ideal value. For example, at visible light wavelengths, atmospheric turbulence will further degrade the telescope resolution. Technology such as adaptive optics can allow telescopes to overcome atmospheric turbulence and approach diffraction-limited resolution.

Telescope resolution can be measured by the angular size of the smallest details that can be distinguished. Objects separated by less than this angle are blurred together and cannot be separated. The angle, represented by the symbol θ (measured in radians), is related to the wavelength of the light, λ, and the telescope diameter D by θ≈λ/D. Small angles θ represent finer details and hence better resolution. For example, ESO’s optical and infrared Very Large Telescope on Cerro Paranal has Unit Telescopes with mirror diameters of 8.2 metres. At infrared wavelengths of about 2 micrometres, they have a diffraction-limited resolution of about 50 milliarcseconds (a little over ten millionths of a degree).

Q: What is the origin of the reddish or greenish glow in the sky that appears in night pictures and time lapse videos taken at exceptional sites such as ESO’s Paranal Observatory?

A: In most cases, the diffuse red or green light is due to a natural faint glow of the atmosphere called airglow. This faint airglow is invisible to the naked eye, but it can be recorded photographically by using long exposure times. However, faint glows in photos and video footage can also be produced by a mix of other sources, both natural and artificial.

Among the natural sources are: sunlight illuminating the high atmosphere a long time after sunset or before sunrise, moonlight from a tiny crescent or waning moon or the zodiacal light, which is sunlight scattered by interplanetary dust inside the Solar System.

Moreover, even at exceptional sites such as ESO’s Paranal Observatory, clouds may occasionally appear. A very thin layer of high altitude clouds can produce a diffuse veil of light originating from the Sun and Moon.

Besides these natural sources, artificial light can also contribute to the glow. Even from isolated sites such as Paranal Observatory, light pollution from very distant cities or mines can be spotted in long exposure pictures or videos. Fortunately it is a small effect, visible only along the horizon.

Q: Is it possible to visit any ESO sites other than the observatories?

A: Unfortunately, ESO currently does not offer public visits to ESO Headquarters in Germany or to the ESO Office in Santiago de Chile. However, interested people can find ESO at various permanent exhibitions, both in Munich and in Chile. For more information, please see: ESO Permanent Exhibitions http://www.eso.org/public/events/exhibitions/perma_exhibitions.html.

Q: Is it possible to get data from ESO or Hubble Space Telescope archives?

Q: Is it possible to buy or name a star?

A: The International Astronomical Union is the internationally recognised authority for naming celestial bodies and surface features on them. Names are not sold, but assigned according to internationally accepted rules. For more information on this topic, please see: http://www.iau.org/public/buying_star_names/.